With the explosion in the growth of Internet usage among both businesses and households, telephone companies have been pressured to provide affordable, high bandwidth access that will support high-speed multimedia services, such as video on demand, high speed Internet access, and video conferencing. To meet this demand, telephone companies are increasingly turning to DSL technology. DSL, while having several different embodiments, can provide throughput rates over 400 times faster than that available through traditional 14.4 kbps modems. For example, the following manifestations of DSL technology are either available today or are currently being tested on a trial basis: Asymmetric Digital Subscriber Line (ADSL), which has a throughput of 32 kbps to 8.192 Mbps downstream to the customer and 32 kbps to 1.088 Mbps upstream to the network; Rate Adaptive Asymmetric Digital Subscriber Line (RADSL), which is a rate adaptive variation of ADSL; High-bit-rate Digital Subscriber Line (HDSL), which offers full duplex throughput at T1 (1.544 Mbps) or E1 (2.048 Mbps) data rates; Symmetric Digital Subscriber Line (SDSL), which provides bi-directional throughput at data rates ranging from 160 Kbps-2.084 Mbps; and Very high-bit-rate Digital Subscriber Line (VDSL), which provides high data rates for customers close to the central office (e.g., 51 Mbps for subscribers within 1000 feet). But most importantly, xDSL offers these high data rates over a standard copper telephone line. Thus, with such a large, embedded copper network already in place, network operators view xDSL technology as a means for extending the life of their investment in copper by many years.
Inasmuch as xDSL is deployed over the copper network, it is susceptible to the same unwanted noise signals that plague traditional copper based communication systems. Noise can be generated by components both internal to the communication system, such as resistors and solid state devices, and sources external to the communication system, such as atmospheric noise, high-voltage power lines and electric motors.
It is well known from information theory that the capacity of a channel (i.e., maximum data rate) is directly related to the logarithm of the ratio of the signal power to the noise power on the channel. Therefore, to support the high data rates associated with xDSL, it would seem desirable to boost transmission power levels to boost the signal-to-noise ratio. As discussed in the foregoing, however, most xDSL systems operate across a broad range of data rates. Thus, if the transmission power level is statically set to support the highest rate possible, this will result in a waste of power for data sessions running at lower throughputs. Moreover, high transmission power levels unfortunately contribute to a phenomenon known as crosstalk, which is perhaps the most common and troubling source of noise in a network.
Crosstalk is defined as the cross coupling of electromagnetic energy between adjacent copper loops in the same cable bundle or binder. Crosstalk can be categorized in one of two forms: Near end crosstalk, commonly referred to as NEXT, is the most significant because the high energy signal from an adjacent system can induce relatively significant crosstalk into the primary signal. The other form is far end crosstalk or FEXT. FEXT is typically less of an issue because the far end interfering signal is attenuated as it traverses the loop. Crosstalk is a dominant factor in the performance of many systems. As a result, xDSL system performance is often stated relative to “in the presence of other systems” that may introduce crosstalk. Therefore, in central office (CO) environments where many xDSL loops or other circuits are bundled together in the same cable binder, it is often desirable to minimize transmit power levels to the lowest levels possible that will still support the desired data rates to reduce the effects of crosstalk between the loops.
Alternatively, where maximum throughput is sought, it becomes desirable to maintain the transmit power level of a given xDSL communication session thereby allowing the data rate to be maximized within the limitations imposed by the noise characteristics of the channel. Optimization of xDSL performance in a central office environment would typically require a combination of both power reduction on some channels and increased throughput or data rates on other channels.
In addition to crosstalk, there may be other reasons to adapt power levels. One of these is to reduce unwanted noise created by the system itself. Certain impairments on the copper loop, such as bridged taps (an unterminated parallel length of wire) may create reflections and distortion energy that can reduce the overall performance of the system. Reducing the power in a frequency band that creates distortion energy or increasing the power in a band that does not create distortion energy can improve the performance of the overall system.
In view of the foregoing discussion, what is sought is an xDSL system and process that dynamically adjust the transmit power levels, data rates, and other defined performance parameters of one or more specific communication sessions to customize overall system performance.